The present invention generally relates to internal combustion engines, and more particularly to modified multicylinder, gasoline burning internal combustion engines wherein an even number of culinders are isolated for the purpose of burning the exhaust products of the engine.
It is well recognized that the combustion of fuel in gasoline burning combustion engines is incomplete resulting in the production of exhaust products rich in unburned hydrocarbons. These exhaust gases represent wasted energy and are a source of pollution. Over the years, varying degrees of interest have been shown in attempts to recover this wasted energy and make the engine more economical to operate and also to reduce as much as possible various exhaust products including unburned hydrocarbons and nitrogen oxides which are serious pollutants.
One approach is illustrated by the patent of Pratt, U.S. Pat. No. 2,113,602, issued Apr. 12, 1938. Pratt discloses an inline eight cylinder engine, the front four cylinders of which are gasoline burning cylinders and the rear four cylinders of which are exhaust burning cylinders. Pratt designates the gasoline burning cylinders as primary cylinders and the exhaust burning cylinders as secondary cylinders. All of the exhaust gases discharged by the primary cylinders, instead of passing into the atmosphere, are collected and treated to provide a new explosive mixture which is then passed to and burned in the secondary cylinders. Pratt requires an air pump or some other means of supplying pressurized air for mixing with the exhaust gases of the primary cylinders. A mixing box is located between the primary exhaust manifold and the secondary intake manifold, and this mixing box is provided with baffles and a flame screen which are necessary for both better mixing and preventing precombustion in the intake manifold of the secondary cylinders.
While Pratt discloses one approach to burning the exhaust products from a gasoline burning engine, his approach has several important drawbacks. The pressurized intake manifold for the secondary exhaust burning cylinders results in a high positive pressure in that intake manifold. This high positive pressure is not conducive to either good induction or correctly controlled fuel/air ratios in the secondary cylinders. Moreover, the arbitrary division of the engine into primary and secondary cylinders necessarily results in unacceptable torsional vibrations because of the large difference in operating efficiencies of the primary and secondary cylinders.
A more recent approach to the general idea of staged combustion in internal combustion engines to obtain low emissions of burned hydrocarbons, carbon monoxide and nitrogen oxides is disclosed in the patent to Siewert, U.S. Pat. No. 3,924,576 issued Dec. 9, 1975. In one embodiment, Siewert uses a V8 engine in which two cylinders of each bank are gasoline burning and the remaining two cylinders are exhaust burning. These first and second stage cylinders correspond to the primary and secondary cylinders in the Pratt system, and like the Pratt system, Siewert employs a pressurized system to supply exhaust from the first stage cylinders to the second stage cylinders. The second stage cylinders use compression ignition instead of spark ignition. The first stage cylinders are provided with a fuel-rich mixture which yields low residuals of nitrogen oxides but substantial unburned hydrocarbon exhaust products. These exhaust products are conditioned by injecting air to provide a slightly lean fuel mixture to the second stage cylinders.
The Siewert approach represents an improvement over the earlier ideas of Pratt, yet there are drawbacks of this method of staged combustion. For example, it is necessary for the first stage fuel ratio to be on the order of 10-1 in order to minimize the production of nitrogen oxides which are produced in some considerable quantity at the stoichiometric ratio of about 15-1. As a result, a two-stage V8 engine built according to the Siewert approach has a substantially lowered power output.